In a nutshell, assuming we're talking about flying the same plane with whichever power system we pick (ie, a Tower Trainer .40 with either a .40 glow engine or comparable electric system)

Glow
Pros: much cheaper (even with fuel), a bit more durable in crashes, no fancy charger needed, the noise is fun, you can fly as much as you want without stopping
Cons: messy, less reliable until you learn how to tune (not hard), power comes on a little slower, usually a tad heavier, requires a field box full of support equipment, more maintainence

Electric
Pros: Simple to get started, no tuning to learn, power is instant, lighter weight if you spend the money on the best components, clean, covering lasts longer and you have more paint options
Cons: Waiting for batteries to charge between flights, a computerized charger is required, much higher startup cost, batteries are fragile and easy to ruin

Now if you're asking about a traditional .40 size trainer vs. a small electric like a Super Cub or Apprentice, then it's apples vs. oranges. The glow plane will definitely fly better, but the smaller size of the electrics lets you fly in planes the bigger planes can't. Obviously the cost differences change dramatically when we're talking about a small electric vs a big glow plane.